1. Field of the Invention
The present invention relates to a method for economically recovering lithium from complex brines containing a significant amount of sulfate, so as to maximize the recovery of lithium values. The present invention particularly pertains to an improved process for recovering lithium sulfate monohydrate from a brine.
2. Description of the Prior Art
A number of naturally occurring brines contain, at least in the commercial sense, a significant quantity of lithium values. The lithium values of some natural brines, such as for example, those found in Clayton Valley, Nev. can conveniently be recovered as precipitated lithium carbonate by standard treatment techniques involving stage-wise evaporative concentration of the brine followed by treatment of the concentrated brine with soda ash. Other brines, however, because of a higher concentration of lithium and sulfate in addition to a large concentration of magnesium and a variety of other constituents resist such treatment. Exemplary of such complex brines are the brines from the Salar de Uyuni in Bolivia, S.A., and the Salar de Atacama in Chile, S.A. A representative composition of such complex brines is set forth below in Table I.
TABLE I ______________________________________ Constituent Weight Percent ______________________________________ Li 0.139 Na 8.06 K 1.84 Mg 0.98 H.sub.3 BO.sub.3 0.36 SO.sub.4 1.80 Cl 15.93 H.sub.2 O 71.00 ______________________________________
Notably, the lithium content of such natural brines typically is about an order of magnitude higher than brines of the Great Salt Lake and Clayton Valley.
The relative concentration of lithium to sulfate in such brines limits the extent to which the brine can be evaporatively concentrated using conventional techniques because lithium tends to precipitate out as a potassium-lithium sulfate double salt (KLiSO.sub.4), at the same time a variety of other salts also precipitate, without substantially improving the concentration of lithium in the brine. The precipitate constitutes a source of lithium loss leading to a lower recovery of lithium in the brine liquor. The precipitate also represents an impurity in the mixed sulfate salts generally recovered from such brines. Normally, these sulfate salts are converted to potash fertilizer using known procedures.
As a rule of thumb, the problem of premature precipitation of lithium as a double salt from a complex brine of the type shown in Table I, i.e., a brine containing lithium, sodium, potassium, magnesium, boron, sulfate and chloride components, can be expected to some extent whenever the lithium content of the brine exceeds about 0.5% by weight and the mol ratio of lithium to sulfate in the brine is between about 0.5 and 1.7. The potassium and magnesium concentrations of the brine also influence when precipitation of the double salt will be encountered.
In U.S. Pat. No. 4,187,163 a process is disclosed, wherein a soluble sulfate salt is used as an agent to precipitate lithium sulfate monohydrate from the complex brine of the type illustrated in Table I. Initially, the brine is evaporatively concentrated to remove halite and sylvite. Then, the brine is chilled to below about 10.degree. C. and preferably about 0.degree. C. to precipitate Epsom salt (MgSO.sub.4.7H.sub.2 O). Some sodium and potassium chloride unavoidably crystallizes with and degrades the purity of the Epsom salt.
After additional evaporation to remove potash salts (and in some cases bischofite) from the brine and to concentrate the brine to at least about 60 mols magnesium chloride per 1000 mols of water and more typically up to about 95 mols magnesium chloride per 1000 mols of water, a soluble sulfate salt is added to the concentrated brine to precipitate lithium sulfate monohydrate. The magnesium chloride content of a brine is determined by assuming that all sulfate in the brine appears as magnesium sulfate and that the remaining metal cations, magnesium included, appear as chloride salts.
Garrett and Laborde, (1980) Salting Out Process for Lithium Recovery also discloses that a second cooling step could be employed after the brine's additional evaporation and prior to treatment with the soluble sulfate salt to precipitate additional Epsom salt, if desired. The lithium-depleted brine then can be recycled to the first chiller or treated, for example, with a strong acid, to precipitate boric acid; while the precipitated lithium sulfate monohydrate is processed for lithium recovery.
While this process permits the recovery of lithium from a complex brine of the type illustrated in Table I, the process steps disclosed for treating the brine prior to addition of the soluble sulfate salt lead to an undesirably low recovery of lithium from the original complex brine. Typically, significant quantities of lithium are coprecipitated with the potash salts when the brine is evaporatively concentrated under normal conditions, for example, when using solar energy. The described process typically is limited to lithium recoveries on the order of about 10-15% from the original complex brine.
It is an object of the present invention to provide an improved process for recovering lithium from complex brines containing relatively high concentrations of lithium, sulfate and magnesium using the technique of common ion precipitation.
It is another object of this invention to provide a process which significantly improves the recovery of lithium from complex brines containing relatively high concentrations of lithium, sulfate and magnesium using the technique of common ion precipitation.
It is another object of this invention to provide a process for recovering lithium in high purity and yield from complex brines containing relatively high concentrations of lithium, sulfate and magnesium, wherein the bulk of the evaporative concentration can be done using solar energy.
These and other objects of this invention will become apparent to one skilled in this art from a consideration of the specification and appended claims.